A vitreous mould composite material is understood to be a reinforcement in three-dimensional structures, the material being composed of ceramic fibers, such as SiC, C, Al.sub.2 O.sub.3, SiO.sub.2, etc., which may be long or short, and a vitreous mould material constituted by at least one standard oxide, such as SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, B.sub.2 O.sub.3, or alkaline or alkalino-earth oxides, etc. These materials are densified by hot isostatic or uniaxial pressing making it possible to embody simple pieces, such as plates or caps.
The document FR-A 2 521 126 describes composite glass structures reinforced with discontinuous fibers of silicon carbide which possess good mechanical resistance, tenacity and stability to oxidation, even at high temperatures, and are thus adapted to the production of these simple pieces.
So as to embody composite items with complex shapes with a glass mould reinforced with fibers, known methods appear to be inadequate. In fact, the reinforcement of composite items by means of continuous fibers and obtained by the use of strips, felt or aligned fiber paper which are infiltrated by glass pulp, cut and then oriented and stacked in a mould for flat pressing, makes it possible to solely obtain a flat arrangement of fibers which cannot be deformed into complex shapes without the fibers being seriously deformed. So as to mitigate this drawback, there is a solution which consists of preimpregnating sheets with reinforcement fibers with a liquid containing a thermoplastic polymer bonding material and glass powder. The sheets are then cut into a particular pattern or model of the complex composite portion to be embodied and are then stacked on preforms which are hot-moulded. Finally, the formed item is heated to decompose and remove the temporary bonding material prior to hot pressing the treated item so as to form the final composite product with high resistance, as described in the document FR-A 2 527 517.
The hot pressing of these materials may be ensured, as demonstrated in the document FR-A 2 658 753, by employing at least one mould element made of a material resistant to high temperatures and which of course possesses mechanical resistance properties, as well as anisotropic thermal conductivity properties, so that the preferential thermal transfer is effected in the pressing oven in the direction of the zones of the piece most distant from the oven heating means.
These methods are therefore suitable to obtain some complex pieces but not all complex pieces and especially are not suitable for obtaining hollow complex pieces incorporating internal stiffeners.
With this aim in mind, the applicant has created a method for making pieces of this type.